Flexographic printing is a method of direct rotary printing that uses a resilient relief image in a plate of rubber or photopolymer to print articles such as cartons, bags, labels or books. Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Photosensitive printing plates and cylindrical printing sleeves have been developed to meet the demand for fast, inexpensive processing and long press runs.
While the quality of articles printed using flexographic plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in a plate remain. In particular, it is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. Some digital flexographic printing elements use an integral UV-opaque mask layer coated over the photopolymer as described for example in U.S. Pat. No. 5,925,500 to Yang et al. and U.S. Pat. No. 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. In a pre-imaging (or post-imaging) step the floor of the plate is set by back exposure to UV light which hardens the photopolymer to the relief depth required for optimal printing. This step is followed by selective ablation of the mask layer with a laser (e.g., infrared laser) to form an image mask that is opaque to ultraviolet (UV) light in non-ablated areas. Next, the print element is exposed to image-forming UV radiation and developed to remove areas not exposed to UV radiation and reveal the relief image, for example using solvents or heat plus blotting. The combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
Solid photocurable elements used to make flexographic relief image printing plates typically comprise a support layer, one or more photocurable layers, and often a protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready to be imaged. The photocurable element may also optionally comprise a slip film disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer.
The use of a photosensitive printing medium for the manufacture of flexographic printing elements, including plates and sleeves, is described in general terms as follows. The photosensitive printing material is deposited onto the support layer to form a printing element, and may be deposited onto the support layer in a variety of ways, e.g., by extrusion, roll coating, heat processing, solvent casting, and the like. These techniques can be readily carried out by those skilled in the art.
The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred substrate materials include steel, copper, or aluminum sheets, plates, or foils; paper; or films or sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. The support sheet can optionally comprise a bonding layer for more secure attachment to the photocurable layer(s) and control of the substrate's reflectivity.
The photosensitive layer(s) can include a variety of known photosensitive materials, such as polymers, initiators, reactive diluents, fillers, and dyes. Preferred photosensitive compositions include an elastomer compound, an ethylenically unsaturated compound having at least one terminal ethylenic group, and a photoinitiator. Such materials are described in numerous patents and publications and are well known to those skilled in the art.
The photosensitive materials of the invention should cross-link (cure) and, thereby, harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and near-UV wavelength regions. Preferred actinic wavelength regions are from about 320 nm to about 450 nm, more preferably from about 350 nm to about 415 nm.
The desired image is produced on the printing plate by exposing selected portions of the resin to actinic radiation. Selective exposure of the photosensitive resin has traditionally been achieved for example, by the use of an image-bearing mask such as a negative film on the surface of the photosensitive layer, through the front side of the photosensitive resin. Areas of the mask opaque to actinic radiation prevent the initiation of free-radical polymerization within the photosensitive layer directly beneath the mask. Transparent areas of the image-bearing mask will allow the penetration of actinic radiation into the photosensitive layer, initiating free-radical polymerization, rendering those areas insoluble in the processing solvent. Alternatively, exposure of selected portions of the photosensitive layer to laser radiation or other digitally controlled actinic light source (i.e., digital platemaking) may also initiate free-radical polymerization, rendering those areas insoluble in the processing solvent.
In the case of digital direct-write plate making or “computer to plate” (CTP) processes, digital files incorporate a variety of text and graphical elements. Digital platesetting interprets and images digital data onto a printing plate that is used as the medium to transfer ink to paper on a printing press. Printing presses include any of a variety of plate printing processes, such as offset, flexo and gravure. In the CTP process, the digitally controlled actinic light is selectively scanned across the dimensions of the plate as guided by an image stored in an electronic data file. The image area of the plate is directly cured by the actinic light, leaving the non-image area uncured and able to be removed in the plate processing step.
Typically, a workstation receives image data. For example, the workstation may be a general purpose programmable computer such as a PC running a MICROSOFT WINDOWS® or Linux operating system. The image data is converted into a raster image by a Raster Image Processor (RIP). “RIPing” is the process of configuring the image file in a way that the device can print it. For example, if the device supports a certain dots per inch (DPI) resolution, the file is “ripped” to that resolution. The RIP may employ a plug-in implanting rasterizing functions in hardware, a stand-alone hardware device, and/or a software module that runs on a suitable computer. The RIP converts the image data into halftone data, and the halftone data is passed onto an image processor. The image processor may be implemented in hardware, software or both. The image processor scans the halftone data and the resulting halftone data is sent to an output device that prepares the flexographic printing plate in accordance with the halftone data. The image processor may process the halftone data on the fly or, more preferably, the workstation may incorporate or allocate memory for storing halftone data prior to outputting it to the imaging device.
Thereafter, a development step is employed to selectively remove the unexposed and therefore unhardened portions of the resin. Development may include, by way of example and not limitation, washing in a suitable solvent or thermal blotting, as is well known in the art. The resulting surface has a relief pattern that reproduces the image to be printed. The printing element is then mounted on a press and printing commences.
Printing plates such as flexographic plates coated with a photopolymer resin layer are typically digitally imaged or patterned using a modulated and rasterized laser beam or beams, or an array of light produced by an illuminated and re-imaged spatial light modulator array, in a machine such as a computer-to-plate (CTP) exposure system. Flexo plates are typically not manufactured in an oxygen-free environment and are not coated or packaged in any way to rigorously exclude molecular oxygen. As such, oxygen is distributed throughout the photopolymer. Oxygen is well known to those skilled in the art to be a strong inhibitor of the radical cure chemistry that is most commonly used in flexo printing plates. If oxygen is removed from such a flexo plate, ambient oxygen will diffuse back into the resin over time if the plate is in contact with atmospheric air.
The third dimension of the exposed and developed flexo plate, especially the sidewall slope of a patterned feature, is a critical determinant of exposed image quality. For an isolated dot on such a plate, a wide base and a narrow flat top are preferred. This is achieved with conventionally exposed ultraviolet sensitive photopolymer resins by first “bumping” the plate, i.e., uniformly flooding the entire plate with ultraviolet light to photometrically consume the dissolved oxygen throughout the resin, followed by exposing with patterned light to polymerize selected features on the plate. The exposed plate is then developed, leaving a residual image of polymerized resin attached to the plate substrate. The bumping process initiates a chemical reaction in the unexposed plate with a time constant that in thick flexographic resins has been measured in several seconds. Immediately after the bump is applied, ambient oxygen begins to diffuse back into the plate from the surface. This leads to a higher oxygen concentration in the resin closer to the surface (top of plate) than to the substrate (bottom of plate) so that photopolymerization inhibition is greater just near the surface. Thus, when the patterning exposure is performed, the dot base is more polymerized than the top resulting in a wider base and a narrower top.
For a successful flexo CTP system, it is necessary to tailor the oxygen concentration throughout the resin cross section with a pre-exposure system that adjusts the bump irradiance and the elapsed time from the bump until the patterning exposure.
Photosensitive resin compositions generally cure through radical polymerization, upon exposure to light. The curing reaction is inhibited by oxygen, which is dissolved in the resin compositions, because oxygen functions as a radical scavenger. It is therefore highly preferred that the dissolved oxygen be removed from the photosensitive resin composition prior to exposure. Various techniques have been suggested for removing dissolved oxygen from the photosensitive resin composition. For example, the photosensitive resin composition may be placed in an atmosphere of inert gas (e.g., carbon dioxide or nitrogen) overnight before exposure in order to replace the dissolved oxygen with the inert gas by way of diffusion. The drawback to this method is that it can take a long time and requires a large space for the necessary machinery.
Alternatively, the photosensitive resin printing element may be given a weak blanket “pre-exposure” to consume the dissolved oxygen prior to subjecting the printing element to the main image-wise exposure. This pre-exposure step is also called a “bump” exposure. The bump exposure is applied to the entire plate area and is a short, low dose exposure of the plate that ostensibly reduces oxygen, which inhibits photopolymerization of the plate (or other printing element). Without this pre-sensitization step, fine features (i.e., highlight dots, fine lines, isolated dots, etc.) are not preserved on the finished plate without unusually long and inconvenient exposure times. However, the pre-sensitization step, if done to excess, can tend to cause shadow tones to fill-in, causing the printed gamut to be significantly reduced. This is exacerbated in plate formulations that have very high sensitivity and small exposure latitude. An additional drawback to this method is that the bump exposure requires specific conditions, including exposure time, irradiated light density, and the like, so that only the dissolved oxygen is quenched.
The pre-sensitization effect also wears off as the elapsed time between the bump exposure and main exposure increases. In conventional exposure of a printing plate, the elapsed time between the bump exposure and the main exposure is typically greater than about 10 or 20 seconds, allowing some oxygen to re-enter the plate prior to the main exposure. This causes the finished plate to have acceptable deep shadows. On the other hand, if the main exposure is applied very soon after the pre-sensitization step, as is envisioned in a computer-to-plate process, the tendency of shadow tones to fill is further worsened in comparison with conventional exposure techniques.
An in-line bumping and exposure system for printing plates and other substrates having a photosensitive layer is described in U.S. Pat. No. 6,903,809 to Donahue et al., the subject matter of which is herein incorporated by reference in its entirety. The system described by Donahue includes a linear illumination source for bumping the photosensitive material with a band of illumination to consume the dissolved oxygen within the photosensitive layer. However, this system, as with other prior art systems, provides a blanket pre-exposure of the entire photosensitive surface. For optimum image quality, the platesetter must typically wait between 2 and 100 seconds to permit some oxygen to return to the plate surface layers but not so long as to allow ambient oxygen to diffuse back into the middle layers of the photopolymer resin. This can be a disadvantage as it can create a time delay and thus lengthen the printing process.
Other efforts to improve the relief image printing plate have involved special plate formulations alone or in combination with the bump exposure.
For example, U.S. Pat. No. 5,330,882 to Kawaguchi et al., incorporated herein by reference in its entirety, describes a photosensitive resin composition that comprises a polymer binder, a radically polymerizable monomer, a sensitizing dye, and a polymerization initiator wherein the preliminary (bump) exposure is conducted with a light that only excites the sensitizing dye and the main exposure is conducted with a light that excites the photopolymerization initiator. In this instance, the preliminary exposure is conducted with a light only exciting the sensitizing dye, and the main exposure is conducted with a light exciting the photopolymerization initiator.
U.S. Pat. No. 4,540,649 to Sakurai, incorporated herein by reference in its entirety, describes a photopolymerizable composition that contains at least one water soluble polymer, a photopolymerization initiator and a condensation reaction product of N-methylol acrylamide, N-methylol methacrylamide, N-alkyloxymethyl acrylamide or N-alkyloxymethyl methacrylamide and a melamine derivative. According to the inventors, the composition eliminates the need for pre-exposure conditioning and produces a chemically and thermally stable plate.
U.S. Pat. No. 5,645,974 to Ohta et al., incorporated herein by reference in its entirety, discloses a photocurable mixture that includes paraffin or a similar waxy substance to inhibit effect of atmospheric oxygen. Due to its low solubility in the polymer, the paraffin floats at the beginning of the polymerization and forms a transparent surface layer that prevents the re-ingress of air once the exposure step is underway.
Although various methods of inhibiting/removing dissolved oxygen in the photosensitive resin composition have been suggested, there remains a need in the art for an improved method of removing dissolved oxygen, especially in computer-to-plate (CTP) processes.